1. The Field of the Invention
The present invention generally relates to optoelectronic components, such as optical receivers. More particularly, embodiments of the invention relate to systems and methods for filtering high frequency noise from a transimpedance amplifier including power supply noise, ground noise, and output common mode noise.
2. Related Technology
Fiber optic cables are one of the mediums used in computer and telecommunication networks. Fiber optic networks use modulated light signals to transmit data. The light is often generated by a laser whose current is modulated to represent the ones and zeroes of the digital data stream. Although light signals are used to carry data over fiber optic cables, the light signals are typically converted into electrical signals in order to extract and process the data. Computers need some type of device to detect and translate the optical signals.
Optical receivers are specifically built for the purpose of receiving and interpreting light signals. An optical receiver typically includes some sort of detector that can generate an electrical current or voltage in response to changes in the power of the incident optical signal. After the fiber optic receiver converts the optical signal received over the optical fiber into an electrical signal, the optical receiver amplifies the electrical signal, and converts the electrical signal into an electrical digital data stream.
One of the common devices used as a detector in an optical receiver is a photodiode. A photodiode operates by generating a current in response to incident light. The optical power of the incident light determines the current that flows in the photodiode. In effect, the optical signal generates current in the photodiode that corresponds to the digital data carried by the optical fiber.
In order to operate properly, the photodiode is reverse biased having its anode connected to the input of the transimpedance amplifier and its cathode to a power supply or a biasing circuit. Ideally, the power supply or biasing circuit provides a constant voltage such that the current in the photodiode is attributable to the incident optical signal. Unfortunately, power supplies almost invariably have noise. The noise may come from switching, host board connections, and other sources. Further, power supply noise often is at a frequency that may be in the same range as the frequency of the data being transmitted on the optical network. As a result, the noise of the power supply can influence the current that is generated by the photodiode and has a detrimental effect on the performance of the optical receiver.
The noise on the power supply enters the transimpedance amplifier with the current generated by the incident light signal. The noise is amplified by the transimpedance amplifier along with the signal from the photodiode. Thus, the performance of the transimpedance amplifier suffers. One potential solution to the problem of power supply noise is to use a decoupling capacitor at the cathode of the photodiode to shunt the noise to ground. Decoupling capacitors are discrete circuit components and therefore introduce additional cost into the production of each optical receiver. Decoupling capacitors are also connected to an external ground that is often different from the ground associated with integrated circuits. This is particularly true for higher frequencies.
Other sources of high frequency noise, such as integrated circuit internal ground noise, and common mode noise are unaffected by the external decoupling capacitor. In transimpedance amplifiers, the local ground of the transimpedance amplifier is often connected to an external ground with, for example, bond wires. At higher frequencies, the inductance of the ground bond wires introduces a significant impedance between the IC internal and external ground and can therefore introduce noise that is amplified by the transimpedance amplifier, thereby reducing the performance of the transimpedance amplifier.
Common mode noise can often enter into the transimpedance amplifier through the output of the transimpedance amplifier. Other integrated circuits that connect to the output of the transimpedance amplifier can introduce common mode noise in this manner. One of the reasons that common mode noise becomes a problem is because the input stage of the transimpedance amplifier is not a differential input. As a result, common mode noise that migrates to the input stage of the transimpedance amplifier can impact the performance of the transimpedance amplifier.